This invention relates to electrical contact materials. More particularly, it is concerned with improving the densities of silver electrical contact materials produced by powder matallurgical methods.
Many types of electrical contacts are fabricated by powder metallurgical techniques. Contacts of certain materials, specifically silver-cadmium oxide, are extremely difficult to attach as by brazing or soldering to copper or brass backing members. In order to permit attaching of contacts of these materials to backing members it is common practice to provide backing layers of fine silver on the primary contact materials. A contact is attached to a backing member by directly soldering or brazing the silver backing layer to the backing member.
Contact materials having a backing layer of silver may be produced by the powder metallurgical technique of double fill pressing. In this technique finely-divided particles of the primary contact material, for example silver-cadmium oxide, are placed in a die cavity. A layer of silver particles is added and both powders are pressed simultaneously. The resulting bi-layer compact is then sintered to produce the final contact material.
In the preparation of silver-cadmium oxide contact materials with silver backing layers, one of the persistent problems has been that the silver layer does not shrink to the same extent as the primary contact material layer. If the shrinkage of the silver backing material and the primary contact material are not closely matched, the sintered piece is warped setting up a highly stressed region at the interface. These stresses can produce cracks, and in subsequent metal working operations, for example coining or rolling, these cracks can propagate causing ultimate failure.
There are also other undesirable effects if the silver backing layer of a contact is of insufficient density. Since the electrical conductivity is relatively low for a silver material of relatively low density, a greater amount of heat is produced by the flow of electrical current; and since the thermal conductivity is also low, heat dissipation through the backing member is low. In addition, mechanical strength and hardness are low. Flux or other impurities may become entrapped in the pores in the backing layer causing brazing problems.
Another problem with silver backing layers is surface blisters. Finely-divided silver powder has certain gases and other impurities entrapped within it. When the silver powder is heated, entrapped gases and volatile impurities evolve. If a compact of silver powder has interconnected pores, the gases and volatile impurities can escape. However, if the surface of the compact has sintered sufficiently to seal off porosity and thus prevent their escape, these gases form blisters at or near the surfaces. The problem becomes more severe with higher compacting pressures.